You will notice two peculiarities: the church of Santa Maria delle Grazie, dating from 1300, has no facade and has an entrance on one side, in the center of three large arches made of trachyte and limestone. Once the most important place of worship in Castelsardo, until the beginning of the sixteenth century it was the cathedral of the diocese that included Anglona. After losing the title to today's spectacular Cathedral of Sant'Antonio Abate, the church became the headquarters of the Oratory of the Brotherhood of Santa Croce. The confreres preserve the tradition of the Lunissanti, the most impressive procession of Holy Week in Castellano. It begins in Santa Maria della Grazie after morning mass and after more than ten kilometers reaches the Basilica of Our Lady of Tergu. The return at night is illuminated by flickering torches.
The interior of Santa Maria consists of a single nave with a neoclassical high altar in the apse, in which, between two niches, there is an ecce homo (Christ sculpture) and other statues of saints. The church is enriched by small baroque wooden altars and a statue of Saint Francis. The cross-vaulted chapel houses the famous Crucifix of the Black Christ from 1300, one of the oldest in Sardinia. It is called Cristu Nieddu because the juniper wood has turned black over time. It was considered a miracle and was carried in procession every time natural disasters struck the city.